Generally, in a flash apparatuses, it is desirable to control the intensity and the colour temperature of the light emitted from the flash tube during a flash as well as the total amount of light emitted from the flash tube.
A flash apparatus typically comprises a generator comprising at least one capacitor configured to apply a voltage to the flash tube. The flash tube comprises a fluid, for example a gas. The fluid in the flash tube is in a normal state an insulator. When a flash is desired, a triggering of the flash tube is performed. There are several methods of triggering the flash tube. For example, a triggering electric current may be supplied to a wire arranged in or around the flash tube. The triggering electric current ionizes parts of the fluid in the flash tube, which causes the fluid to start conducting electric current between the electrodes of the flash tube. The electrical contact between the electrodes of the flash tube allows the at least one capacitor to discharge causing a pulse of electric current to be led through the fluid in the flash tube. This pulse of electric current causes the fluid in the flash tube to emit light. The emitted light has a certain intensity and a certain colour temperature at each point in time during the flash. In addition, a total amount of light is emitted during the flash. The characteristics of the emitted light is dependent on the energy supplied to the flash tube.
The colour temperature of the total amount of light emitted from the flash tube during a flash is an important factor when the flash is used for photographic purposes. In the beginning of the flash, that is, in the beginning of the discharge of the capacitor, normally, light with a higher colour temperature is emitted from the flash tube. At the end of the flash, that is, at the end of the discharge of the capacitor, light with a lower colour temperature is emitted from the flash tube. It is desirable to be able to control the colour temperature of the flash.
In addition, or alternatively, it is desirable to control the intensity of light emitted during a flash and/or the total amount of light emitted from the flash tube during a flash.
One method of controlling the total amount of light and the intensity of the light emitted from a flash tube is to adjust the charging voltage of the capacitors of the generator. The lower the voltage applied to the flash tube, the lower the total amount of light and the intensity of the light emitted during the flash. However, the adjustment of the charging voltage also affects the colour temperature of the emitted light.
One method of controlling the characteristics of the light emitted from the flash tube during a flash is to cut the electrical current supplied to the flash tube at a certain time before the capacitors have discharged completely. By cutting the current to the flash tube at a certain time, the flash tube will stop emitting light and hence, the characteristics of the light emitted from the flash tube may be controlled. With this method, for example a change in colour temperature of the light emitted from a flash tube during a flash caused by a change in charging voltage of the capacitors may be compensated for by cutting the electrical current to the flash tube at a certain point in time.
One way to determine a point in time when the electrical current to the flash tube should be cut in order to achieve the desired characteristics of the light emitted from the flash tube during a flash is to analyse the light emitted by the flash by means of an optical sensor. The data from the optical sensor may then be used in order to determine for example when a certain amount of light has been emitted from the flash. At the moment when a required amount of light has been emitted, the current to the flash tube should be interrupted.
One problem associated with this technique is the requirement of an optical sensor for analysing the light emitted by the flash tube, increasing the cost and the complexity of the flash apparatus.
Another problem is the inaccuracy in optical sensors, introducing a high uncertainty into the determination of the starting point in time for light emission.
Alternatively in the prior art, in order to control the characteristics of the light emitted during a flash, a certain duration of the period of time when light is emitted from the flash tube and hence, a certain duration of the period of time when current is allowed to flow from the capacitor to the flash tube, is required. In order to be able to control the duration of the period of time before the current to the flash tube should be interrupted, a starting time for the flash needs to be determined.
According to prior art, the point in time when a triggering command is sent to a triggering circuit and hence, a triggering current is assumed to start to flow in a triggering circuit, is assumed to be the starting time of the flash and hence is used for determining a point in time when to cut the current to the flash tube. However, there is a variable delay between the point in time when the triggering current is starting to flow until the starting time of the flash due to factors such as temperature, the pressure of the fluid in the flash, triggering energy etc. The duration of the delay is hard to predict due to the many factors involved. Normally this delay is ignored or estimated to be a constant value and the starting time for the flash is assumed to be the starting time for the triggering current, possibly with the addition of an estimated delay. For flashes with a relatively long duration, small errors when determining the starting time of the flash may be acceptable and, therefore, the delay between the triggering of the flash and the actual starting time for the flash may sometimes be neglectable. For flashes with a short duration, however, the accuracy in starting time for the flash determination is crucial and, therefore, the starting time for a flash cannot reliably be determined from triggering current measurements.
Consequently, there is a need and desire for improved control of the characteristics of light emitted during a flash.